The present invention is generally directed to the plasma cutting and more particularly directed toward a method and apparatus used in a contact start plasma cutting process.
There are several known methods of initiating a plasma arc discharge and starting an arc plasma torch (for plasma cutting). The better known include: high frequency or high voltage discharge, contact starting, and with an exploding wire. In each method an arc is drawn between a cathode and an anode, and an ionizable gas is directed to flow around the arc, creating a plasma jet.
The high frequency discharge or high voltage spark discharge method of initiating a plasma arc is relatively old and at one time widely used. The method entails using a high voltage to break down the gap between a cathode and an anode, thus generating charge carriers which create the electric current path necessary to start the arc. Such a method is disclosed in U.S. Pat. No. 3,641,308, to R. Couch, Jr., et al. As disclosed by R. Couch, et al. a brief high voltage pulse provided to the cathode initiates an arc discharge across the gap from the cathode to a grounded workpiece.
However, the high frequency method of arc starting can produce electromagnetic interference in nearby electronic equipment, thus requiring either shielding or a remote location of the high frequency electronics. Furthermore, the equipment required to generate the high frequency discharge may be expensive.
An electrical conductor is extended from the cathode to the workpiece in the "exploding wire" technique. The conductor vaporizes when the current is initiated, leaving the arc in its place. Obviously, the exploding wire technique cannot practically be used in start and stop type plasma cutting processes.
Contact starting of plasma arcs entails touching an anode and a cathode, thus requiring relatively little current and voltage, and eliminating the need for high frequency equipment (along with the associated high cost and electromagnetic interference). The cathode is manually placed into electrical connection with the workpiece in older methods of contact starting and a current is passed from the cathode to the workpiece. The arc is struck by manually backing the cathode away from the workpiece. Often, the cathode is the electrode and the nozzle through which the plasma jet passes serves as an electrical conductor connecting the electrode with the workpiece. The nozzle slides with respect to the electrode, and is forced into contact with the electrode when it is pressed against the workpiece. Thus, the electrode, nozzle, and workpiece function electrically in series when the current flow is initiated. When the electrode is manually backed away from the workpiece, the nozzle is allowed to separate from the electrode and return to its normal position.
One disadvantage of such contact starting systems is that when the nozzle is pressed against the workpiece there is a risk of damaging a brittle ceramic element usually located at the end of the nozzle. Also, it is difficult in practice to initiate a cut while at the same time attempting to press the nozzle down onto a workpiece. Another problem with this starting method is that nonconductive coatings such as paint make electrical contact starting using the workpiece difficult. As a result, a pilot arc circuit may be required, even when contact starting is available.
A more recent type of contact starting torch has a cathode and an anode in the torch that are initially touching. This contact is a path through which current flows. The cathode is then automatically moved and separated from the anode in response to a build up of gas pressure within the torch. The current flowing from the cathode to the anode before the separation creates a pilot arc across the gap as the cathode and the anode separate.
U.S. Pat. No. 4,791,268, to N. Sanders, et al., describes such a torch having a movable electrode acting as the cathode and a fixed nozzle acting as the anode. A spring forces the electrode into contact with the nozzle when no gas is flowing within the torch. In this position the electrode blocks the nozzle orifice. After electrical current begins to flow from the electrode to the nozzle, gas is supplied to the torch. The gas exerts a force upon the piston part counteracting the force exerted by the spring, and, when high enough, the moves them electrode away from the nozzle. This breaks the electrical contact between the electrode and the nozzle and creates the pilot arc. Also, as the electrode moves away from the nozzle, it opens the nozzle orifice, and a plasma jet is provided by the torch.
A torch commercially available today from Hypertherm, Inc., Hanover, N.H., is a contact start torch. The torch has an internal contact mechanism with an electrode to tip shorting position and an open position. The electrode is spring loaded into the shorting position, and may be moved to an open position by means of force applied with compressed air. This contact mechanism provides a reliable pilot current path when shorting and when the contact moves to the open position an arc is created. There is a predetermined travel distance between the shorting and open positions.
The cutting process is initiated with a pilot arc between the tip and electrode. An inductor located in the pilot current path stores inductive energy due to the pilot current. The short is forcibly opened by an applied air flow. When the short is opened, the inductor causes a discharge through the opening gap between the electrode and tip. The energy discharged ionizes the air in the gap, lowering gap resistance, thus providing a path for continuation of pilot current flow (now an arc).
Cutting of metal is initiated by transferring a portion of the pilot arc current from the electrode, through the metal being cut, to the positive polarity terminal of the power source. Electronics in the power source sense when the arc has transferred and then supply a greater magnitude main cutting current after the transfer has occurred. Also, the torch tip is disconnected (electrically) interrupting the pilot current path. Thus, the current is used to cut the workpiece, and follows a path including the positive terminal, the workpiece, and the electrode.
However, this type of torch has a significant drawback: if the arc is extinguished (or does not transfer) the process can only be reinitiated by releasing and retriggering (recycling) a trigger switch on the torch. This disadvantage is of particular importance when cutting an expanded metal (such as a grille), which necessarily involves extinguishing of the arc. Moreover, the cutting arc cannot be reignited until the air pressure built up in the hose leading to the torch is dissipated. This takes some time in the prior art systems, which do not provide a mechanism to vent the hose. Accordingly, a torch and power supply that allows arc reignition without recycling the trigger is desired.
One potential danger of plasma cutting systems is the possibly lethal voltage levels associated with this process. Generally, plasma cutting systems provide safety provisions such as a parts in place (PIP) circuit that will inhibit power source operation and prevent application of a high OCV if any part is missing. This technology does not provide a redundant safety system. Accordingly, it is desirable to provide a redundant safety system that prevents dangerously high open circuit voltages, even if the PIP system is defeated and the torch engaged.
Another shortcoming of known torch and plasma cutting systems is that the torch and consumable parts in the torch can get very hot during operation. Moreover, when the arc is extinguished, the heat is typically not dissipated, thereby shortening parts life and possibly damaging the torch. Accordingly, a torch that provides postarc cooling is desired. However, the cooling should not interfere with reignition of the arc.